1. Technical Field
The invention relates generally to apparatus for forming molten plastic, such as extrusion and injection molding machines, and more particularly to apparatus for adding a colored pigment to the plastic formed by such extrusion or injection molding machines. The invention also relates to apparatus including a barrel and a screw for discharging colored molten material under pressure.
2. Related Prior Art
It is generally known to add a colorant or pigment or other additive to a supply of plastic beads or pellets to be formed through either extrusion or injection molding, both of which entail the liquification of the supply of solid beads into a flow of molten plastic. The liquification of the plastic is accomplished by heating the plastic as a screw moves the plastic along a cylindrical barrel of either an extrusion machine or an injection molding machine. The plastic beads are melted by the high pressure, and resultant frictional forces, created between the plastic, the screw and the walls of the barrel, and by heating elements in the walls of the barrel.
One known method of coloring such a supply of plastic beads is to supplement the supply with a quantity of pigment concentrate. For example, it is generally known to meter liquid pigment into a hopper containing a supply of plastic beads to be introduced to the barrel of an extrusion or injection molding machine. In such an arrangement, the pigment concentrate and the beads are mixed within the confines of the hopper. One of the problems associated with this method of coloring a supply of plastic is that it entails the mixing of solid objects, namely the individual solid beads, with a liquid. Due to the generally spherical shape and unabsorbent nature of the beads, the liquid pigment is not uniformly distributed about the individual beads and is easily wiped off a bead as the bead passes into the barrel.
The amount of pigment that can be introduced to a supply of beads is limited by the interaction of the beads with the screw in the barrel of the forming apparatus. In particular, if an excessive amount of pigment is mixed with the plastic beads in the hopper, the pigment can act as a lubricant between the screw and the beads. By eliminating friction between the face of the screw and the beads, the lubricating effect of the pigment can cause the screw to merely churn the plastic beads rather than advance the beads along the barrel.
Also, the introduction of colored pigment to a hopper prior to the introduction of the plastic into the barrel creates a supply of "contaminated" plastic which, if a different colored plastic is desired, must be purged from the plastic forming apparatus. To purge the contaminated plastic requires the entire hopper of plastic to be either removed from the apparatus or cycled through the apparatus.
Another method of introducing colored pigment to a supply of plastic beads entails the addition of beads of pigment concentrate to the plastic beads. The beads of concentrate are introduced to the plastic beads in a hopper, and the mixed beads then flow into the barrel of the forming apparatus. When the pressure inside the barrel becomes sufficiently intense, the beads of concentrate burst, thereby further mixing with the plastic beads. Such a method of using pigment concentrate in the form of a beads requires the mixing of the beads of plastic and concentrate in a hopper prior to their introduction to the barrel, and the purging of the plastic in the hopper and the barrel to remove contaminated plastic from the forming apparatus. Also, the metering of concentrate in the form of beads is somewhat less precise than that of liquid concentrate.
Another method for introducing colorant to a supply of plastic is to supply liquid plastic concentrate directly to the barrel of the forming apparatus. For such a method to be successful using prior art plastic forming apparatus, the introduction of pigment to the barrel requires high pressure pumps to introduce the pigment to the barrel at a point where the plastic contained by the barrel has begun to melt. High pressure pumps are required in known arrangements because the region of the barrel into which the pigment is injected contains a supply of molten plastic under high pressure, and high pressure pumps are necessary to overcome the high pressure of the molten plastic. The introduction of pigment into the barrel at a point of lower pressure has heretofore been unsuccessful because of the above-described lubricating effect of liquid pigment on the supply of plastic beads. One advantage of introducing liquid pigment to the barrel of a plastic forming apparatus is the superior mixing of the pigment concentrate with the plastic supply. This superior mixing is realized due to the liquid/liquid nature of the two media, rather than the liquid/solid nature of the concentrate and beads. Also, the introduction of the pigment to the plastic supply at a relatively downstream point in the barrel minimizes the amount of contaminated plastic which must be purged in order to change the color of the resultant plastic. However, the expense of high pressure pumps and the controls required to practice this method of introducing colorant to a plastic supply counterbalances these advantages.